The present invention relates to an image forming apparatus which can form color images using developers (developing agents) of a plurality of different colors.
Conventionally, image forming apparatuses have been known which sequentially superimpose toner images of four colors on a belt with use of one photoconductor and collectively retransfer these toner images to a sheet to form color images. In color printing, the apparatuses of such configuration need time to print one sheet since development is performed by switching developing devices so that the developing devices face the photoconductor. However, in monochrome printing, the apparatuses can achieve printing speed similar to that of monochrome printers since the switchover is not necessary, and further since requiring only one photoconductor contributes to low cost, the apparatuses are highly advantageous for users who usually print monochrome images.
Now, in the business field, images which are not in full color but not in monochrome either are often printed. These printing includes, for example, accent colors used in a part of text images, and identification colors such as for use in color graphs inserted into a part of text data images. Such images are mainly used for identification by color and do not require faithful reproduction of delicate colors such as seen in photography data. However, even in such cases, color regions are subjected to image formation process performed by using all four colors of developers. Consequently, the image formation operation of one sheet takes a long time, which in turn causes problems such as wasteful consumption or exhaustion of consumable goods including developers and photoconductors.
Moreover, even when the consumption of the developer of a certain specific color is smaller than the developers of other colors, the image formation process with the developer of the specific color is still performed, and so users may find the operating sound generated at the time of the image formation process unpleasant.
Accordingly, a technique to solve the above-mentioned problem has been proposed, for example, in JP 9-163169A, in which a developing device dispensed from image formation is generated by approximating all the colors reproduced by the image forming apparatus to a total of six colors which are produced from one or two toners.
However, the above-mentioned technique has a defect, that is, for example, those close to red color are all approximated to the same red color, with a result that a graph which should essentially be reproduced in different colors is expressed in the same color, making it impossible to identify the difference.